1. Field of the Invention
The present invention relates to a scanning image forming lens and an optical scanner using this scanning image forming lens.
2. Description of the Related Art
In an optical scanner, a light beam is deflected at an equal angular velocity and is converged as a light spot on a scanned face by a scanning image forming lens so that the scanned face is optically scanned. Various kinds of optical scanners are widely known in association with an optical printer, a digital copying machine, etc.
Recently, a reduction in cost of each of the optical printer and the digital copying machine has been required to spread the optical printer and the digital copying machine. Therefore, a reduction in cost of the optical scanner and compactness of the optical scanner are required.
In the optical scanner, a main portion of the cost of the optical scanner is occupied by a scanning image forming lens. Compactness of the scanning image forming lens is indispensable to the compactness of the optical scanner.
It is effective to construct the scanning image forming lens by a cheaper material and a smaller number of lenses to realize the reduction in cost of the scanning image forming lens and the compactness of the scanning image forming lens. For example, if the scanning image forming lens can be constructed by a single plastic lens, there are no problems in cost and compactness of the scanning image forming lens. However, it is difficult to really realize optical performance required for the scanning image forming lens by the single plastic lens.
Therefore, it is considered that the scanning image forming lens is constructed by two plastic lenses as the second best policy for the reduction in cost and compactness.
An f.theta. lens shown in Japanese Patent Application Laying Open (KOKAI) No. 1-92715 is an example in which the scanning image forming lens is constructed by such two plastic lenses.
This f.theta. lens is constructed by a biconvex lens and a lens having a toric face on an incident side and a spherical surface on an emitting side. The biconvex lens and the lens having the toric face are arranged in proximity to each other so that the f.theta. lens is compact.
The plastic lens can be manufactured in large quantities by injection molding so that manufacturing cost of the plastic lens can be reduced. It is preferable to uniform a shape of the plastic lens as much as possible so as to form a lens conforming to the shape of a die by the injection molding. Namely, the shape of the plastic lens is desirably formed such that there is no great change in thickness of the plastic lens.
In consideration of such a viewpoint, the f.theta. lens shown in the above laid open patent is constructed by a convex lens and a difference in lens thickness between central and peripheral portions of the convex lens is large. Inclining directions of both lens faces are opposed to each other. Therefore, it is difficult to accurately form the f.theta. lens as molded by injection molding.
Further, optical characteristics of the plastic lens tend to be changed by a change in temperature. However, this problem is not considered in the case of the above f.theta. lens.